The present invention relates to a distributed feedback semiconductor laser comprising an active layer sandwiched between the upper side and lower side cladding layer and a diffraction grating formed along said active layer. More specifically, the present invention relates to a distributed feedback semiconductor laser characterized by its structure to enable stable oscillation in the single mode and in addition enable high output operation.
In recent years, trunk line optical communication systems that enable transmission of a large amount of data for a long distance have been established. Therefore, a light source of such optical communication is requested to satisfy both excellent single wavelength oscillation as well as high output operation.
For a light source to realize the above requested characteristics, a distributed feedback semiconductor laser (hereinafter, referred to as DFB laser), particularly a {fraction (xcex/4)} phase-shifted DFB laser has been used. This {fraction (xcex/4)} phase-shifted DFB laser is provided with a diffraction grating having a projected and recessed shape along an active layer and having a {fraction (xcex/4)} phase shift region near the center of the laser resonator. The active layer is formed within an optical waveguide in the resonator.
However, in the refractive index coupled DFB laser explained above, it is difficult to simultaneously attain stability of single mode oscillation and high output operation, and resistance for the reflected beam returning from external sides is rather small. Therefore, a gain coupled DFB laser has been proposed as a DFB laser to overcome the problems of the refractive index coupled DFB laser explained above, and development of such DFB laser is now continued.
In the gain coupled DFB laser, mode selectivity of an oscillation wavelength is enhanced by adding a periodical perturbation of the gain in the guiding direction of an optical beam. Therefore the stable single mode oscillation is possible even if there is no phase shift region and no anti-reflection film at the end surface of the resonator in the refractive index coupled DFB laser.
Moreover, it is experimentally confirmed that a large resistance is also assured for the reflected beam returning from the external sides.
Moreover, since stable single mode oscillation is possible without any anti-reflection films, the anti-reflection film and the high-reflection film can be provided at both end surfaces of the laser. Therefore, high output operation can be realized.
Here, a first conventional art gain coupled DFB laser that can be assumed to have relationship with the present invention will be explained with reference to FIGS. 1(a)-1(d).
FIG. 1(a) is a cross-sectional view of the gain coupled DFB laser of the first conventional art along the guiding direction of an optical beam. In this figure, numeral 31 designates an n-type InP cladding layer; 32, an active layer comprising a multiple quantum well (MQW); 33, a p-type InP cladding layer; and 34, n-type InP current blocking portions. A curve given the arrow mark schematically indicates the flow of a current. FIGS. 1(b) to 1(d) respectively illustrate distributions of gain, refractive index and optical field intensity corresponding to the cross-sectional view of FIG. 1(a) along the guiding direction of an optical beam.
In the DFB laser of FIGS. 1(a)-1(d), the n-type portions 34 are periodically formed within the p-type cladding layer 33. The n-type portions 34 keep a constant distance from the active layer 32 formed in the uniform thickness. Since this n-type portion 34 is embedded in the p-type layer 33, it plays a role of the current blocking layer.
Since a current is pinched in the regions between respective current blocking portions with the effect of this n-type current blocking portion 34, as schematically illustrated in FIG. 1(a), distribution of density of the current flowing into the active layer 32 is lowered in the regions of the active layer just under the current blocking portions 34 and is raised, on the contrary, in the regions where there are no current blocking portions 34 thereon.
This is the reason why the gain of the DFB laser of FIG. 1 is periodically distributed as illustrated in FIG. 1(b). Therefore gain coupling occurs and the laser starts to operate as the gain coupled DFB laser.
Next, a second conventional art gain coupled DFB laser that is assumed to have relationship with the present invention will be explained with reference to FIGS. 2(a)-2(d).
FIG. 2(a) is a cross-sectional view of the gain coupled DFB laser of the second conventional art along the guiding direction of an optical beam. The elements like those of FIG. 2 are designated with the like reference numerals. As in the case of FIG. 2, FIGS. 2(b) to 2(d) respectively illustrate distributions of gain, refractive index and optical field intensity corresponding to the cross-sectional view of FIG. 2(a) in the guiding direction of an optical beam.
In the DFB laser of FIG. 2, the active layer itself is periodically etched to form periodical projected and recessed shapes as illustrated in FIG. 2(a). Therefore, in this shape, thickness of the active layer is periodically changed along the guiding direction of an optical beam. In the region (projected region) where the active layer is thick, the generated gain is larger than that of the thin region (recessed region).
Accordingly, since the periodical gain distribution is formed as illustrated in FIG. 2(b) in the DFB laser of FIG. 2, gain coupling occurs and the laser starts to operate as the gain coupled DFB laser.
A third conventional art gain coupled DFB laser that is assumed to have the relationship with the present invention will be explained with reference to FIG. 3.
FIG. 3 is a cross-sectional view of the gain coupled DFB laser of the third conventional art along the guiding direction of an optical beam. The elements like those in FIGS. 1(a)-1(d) and FIGS. 2(c)-2(d) are designated with like reference numerals. Numeral 35 designates embedded portions.
The DFB laser of FIG. 3 has a structure almost similar to the DFB laser of the second conventional art of FIGS. 2(c)-2(d). However, it is different only in the point that the recessed region of the active layer 32 having periodical projected and recessed shapes is embedded with p-type InGaAsP. This p-type InGaAsP is the quaternary compound semiconductor material having a band gap which is smaller than that of the p-type InP cladding layer 33, namely the refractive index which is larger than that of such material.
Moreover, in the DFB laser of FIG. 3, distribution of the refractive index in the guiding direction of an optical beam is kept small by adjusting a composition of the InGaAsP embedded portions 35 so that the refractive index of the embedded portions 35 becomes close to the average refractive index of the active layer 32.
However, the gain coupled DFB laser structures of the first to third conventional arts explained above respectively have the following problems.
First, the DFB laser of the first conventional art has a problem that it is actually difficult to increase the ratio of gain coupling to refractive index coupling. This will cause a generation of a fluctuation in the single wavelength characteristic of the laser oscillation thereby lowering the stability of the single mode oscillation.
Namely, in the process of forming a groove for current blocking portions 34 by etching the p-type cladding layer 33, it is required to keep a constant margin for the remaining thickness of the cladding layer 33 in order to prevent the active layer 32 from being etched. Therefore, it is actually difficult to reduce the distance between the current blocking portion 34 and the active layer 32.
Therefore, the pinching effect of currents by the current blocking portions 34 becomes insufficient and a current disperses into the regions just under the current blocking portions 34 as illustrated in FIG. 1(a). Accordingly, it is impossible to provide a sufficient difference in injected current density between the regions where there are no current blocking portions 34 thereon and the regions just under these current blocking portions. Therefore, sufficient gain distribution cannot be formed in the guiding direction of an optical beam and thereby a large element of gain coupling cannot be obtained.
Moreover, in the DFB laser of the first conventional art, in addition to the amount of gain coupling, the refractive index coupling also exists in such a degree that cannot be neglected.
Namely, as explained above, a larger amount of current is injected into the regions of the active layer where there are no current blocking portions 34 thereon in comparison with the regions just under the current blocking portions. Thereby the carrier density becomes larger in the regions where there are no current blocking portions 34 thereon.
As a result, the refractive index in the regions where there are no current blocking portions 34 becomes smaller than that in the regions just under the current blocking portions due to the plasma effect. Therefore, as illustrated in FIG. 1(c), since the periodical distribution of the refractive index is also generated simultaneously with the periodical distribution of the gain, the refractive index coupling is also generated simultaneously.
Therefore, in the DFB laser of the first conventional art, it is difficult to realize larger gain coupling to the refractive index coupling.
Moreover, the DFB laser of the first conventional art has a problem that the oscillation efficiency for the injected currents cannot be raised and thereby a threshold current increases.
Namely, in general a standing wave of a optical field in the laser resonator is generated to a large degree within the regions having higher refractive index. Therefore, when the refractive index distribution exists, it is preferable that the phase of a refractive index distribution is coincident with the phase of a gain distribution in order to realize high efficiency oscillation with equal current injection.
However, as illustrated in FIGS. 1(b), 1(c), the gain distribution and refractive index distribution are generated in the inverse phases in the gain coupled DFB laser of the first conventional art. Therefore, the amplification efficiency in the optical field in the resonator is rather bad and the oscillation efficiency for the injected currents cannot be increased. Therefore, it is impossible to avoid an increase of the threshold current.
Next, the DFB laser of the second conventional art has a problem that a threshold current increases since current injection efficiency into the thick regions of the active layer becomes lower.
Namely, in the case of the gain coupled DFB laser of the second conventional art, since the recessed region of the active layer 32 is embedded with the p-type InP layer that is the same material as the p-type cladding layer 33, a potential barrier for a hole is not generated over the recessed region of the active layer 32. Thereby a hole current injected from external sides is ordinarily injected, in almost the same rate, into the thick and thin regions of the active layer having the projected and recessed shape.
In general, since a current injected into the thin regions (recessed regions) of the active layer almost does not contribute to the laser oscillation, approximately 50% of the injected currents is in principle wasted without any contribution to the laser oscillation. Therefore, in the DFB laser of the second conventional art, current injection efficiency into the thick regions contributes substantially to the lowering of the oscillation and a threshold current inevitably becomes large.
Moreover, the refraction index coupling cannot be neglected in addition to the gain coupling in the DFB laser of the second conventional art, as in the case of the first conventional art. Therefore, a problem arises that it is very difficult to sufficiently enlarge the ratio of gain coupling to refractive index coupling. Thereby, fluctuation is generated in the single wavelength characteristic of the laser oscillation and the stability of the single mode oscillation will be lowered.
Namely, the InP cladding layer having a band gap that is larger than that of the active layer, namely having the smaller refractive index, is embedded on the recessed region of the active layer that is periodically etched. Therefore the gain distribution is obtained as explained above and simultaneously the periodical distribution of the refractive index is also generated as illustrated in FIG. 2(c). Thereby a large element of refractive index coupling is also generated.
Particularly, in the case of the gain coupled DFB laser of the second conventional art, when the recessed region of the active layer 32 is set deeper to increase a difference of the film thickness between the thick and thin regions of the active layer 32 and to attain a large element of the gain coupling, the InP layer having the refractive index that is smaller than the average refractive index of the active layer 32 is embedded on the recessed region by amounts proportional to the increase of the depth of the recessed region. Thereby the refractive index coupling also simultaneously becomes large.
Therefore, in the DFB laser of the second conventional art, sufficiently large ratio of the gain coupling to the refractive index coupling cannot be obtained.
Meanwhile, in the DFB laser of the third conventional art, the setting is made so that the amplitude of the refractive index distribution generated in the guiding direction of an optical beam becomes small by adjusting a composition of the InGaAsP embedded portions formed on the recessed region of the active layer. Therefore the refractive index coupling can be reduced to a small value that can be neglected for the gain coupling. Therefore, a certain improvement is apparent in this point.
However, the DFB laser of the third conventional art also has a problem that the threshold current becomes large because current injection efficiency into the thick regions of the active layer becomes lower, as in the case of the second conventional art.
Namely, the recessed region of the active layer 32 is embedded with the p-type semiconductor layer of the same conductivity type as the p-type InP cladding layer 33, particularly with the p-type InGaAsP layer 35 having a band gap that is smaller than that of the p-type InP cladding layer. Therefore any potential barrier for a hole is not generated over the recessed regions and, on the contrary, the dip of potential is formed.
Therefore, since a hole current injected from external sides is injected, in a larger ratio, into the thin regions (recessed region) than the thick regions (projected region) of the active layer 32 having projected and recessed shape, about 50% or more of the injected current is wasted without any contribution to the laser oscillation.
Therefore, in the DFB laser of the third conventional art, since a current injection efficiency into the thick regions contributes substantially to the lowering of the oscillation, the threshold current becomes larger.
The present invention has been proposed considering the problems explained above. It is a general object of the present invention to provide a structure of a gain coupled DFB laser that assures a larger ratio of the gain coupling to refractive index coupling and attains a lower threshold current by generating gain coupling and refractive index coupling in-phase.
Another and a more specific object of the present invention is to provide a DFB laser comprising: a first cladding layer; a second cladding layer having a conductivity type opposite to that of said first cladding layer; an active layer sandwiched between said first cladding layer and said second cladding layer, having periodically projected and recessed surfaces and having thickness which periodically changes, and; current blocking portions formed on said recessed surfaces of said active layer for pinching a current flowing into said first and second cladding layers in order to selectively guide the current through the projected surfaces of said active layer.
With this structure, the thickness of the active layer can be changed periodically, and a greater part of a current injected from external sides can be selectively injected to the thick regions (projected regions) of the active layer, and the amount of a current to be injected to the thin regions (recessed regions) of the active layer can be extremely reduced.
Therefore, since the gain coupling to the refractive index coupling can be increased more than that of the conventional arts, stability of the single mode oscillation of the laser can further be improved. Moreover, oscillation efficiency for the injected current can be improved and thereby a threshold current can also be lowered than that of the conventional arts.